Java Program for Range Queries for Frequencies of array elements

Last Updated : 06 Jan, 2022

Given an array of n non-negative integers. The task is to find frequency of a particular element in the arbitrary range of array[]. The range is given as positions (not 0 based indexes) in array. There can be multiple queries of given type.
Examples:

Input  : arr[] = {2, 8, 6, 9, 8, 6, 8, 2, 11};
         left = 2, right = 8, element = 8
         left = 2, right = 5, element = 6      
Output : 3
         1
The element 8 appears 3 times in arr[left-1..right-1]
The element 6 appears 1 time in arr[left-1..right-1]

Naive approach: is to traverse from left to right and update count variable whenever we find the element.
Below is the code of Naive approach:-

Java

// JAVA Code to find total count of an element 
// in a range 
  
class GFG { 
      
    // Returns count of element in arr[left-1..right-1] 
    public static int findFrequency(int arr[], int n,  
                                int left, int right, 
                                      int element) 
    { 
        int count = 0; 
        for (int i = left - 1; i < right; ++i) 
            if (arr[i] == element) 
                ++count; 
        return count; 
    } 
      
    /* Driver program to test above function */
    public static void main(String[] args)  
    { 
        int arr[] = {2, 8, 6, 9, 8, 6, 8, 2, 11}; 
        int n = arr.length; 
       
        // Print frequency of 2 from position 1 to 6 
        System.out.println("Frequency of 2 from 1 to 6 = " + 
             findFrequency(arr, n, 1, 6, 2)); 
       
        // Print frequency of 8 from position 4 to 9 
        System.out.println("Frequency of 8 from 4 to 9 = " + 
             findFrequency(arr, n, 4, 9, 8)); 
          
    } 
  }  
// This code is contributed by Arnav Kr. Mandal. 

Output:

 Frequency of 2 from 1 to 6 = 1
 Frequency of 8 from 4 to 9 = 2

Time complexity of this approach is O(right – left + 1) or O(n)
Auxiliary space: O(1)
Please refer complete article on Range Queries for Frequencies of array elements for more details!